1. Field of the Invention
The present invention relates to a belt drive system that transmits a rotational power with the use of a belt at the time of starting an engine and at the time of driving an auxiliary machine by means of the engine.
2. Description of the Related Art
As a first prior art of this type, the Japanese Patent Publication (unexamined) No. 14145/1996 discloses a belt drive system in which a crank pulley mounted on a crankshaft of an engine, pulleys respectively mounted on each of auxiliary machines located around the engine, and a pulley mounted on a starting motor are connected through a belt, and the engine is started through the belt by the starting motor as well as each auxiliary machine is driven through the belt by the engine after the engine has been started.
However, since a large transmission torque is required when the engine is started through the belt by the starting motor, it is necessary to apply a high initial tension to the belt. Consequently, a problem exits in that this application of the high initial tension is disadvantageous in the aspect of durability of the belt.
In addition, in the ordinary belt drive system, a crank pulley mounted on a crankshaft of an engine and pulleys mounted on respectively each auxiliary machine are connected through a belt, as well as the crankshaft is driven directly via a gear by a starter when starting the engine, and each auxiliary machine is driven through the belt by the engine after the engine has been started. In such an ordinary belt drive system, it has been known to dispose or insert a first auto-tensioner 20 on the slack side of the belt of the crank pulley 2 in order to reduce initial tension of the belt, as shown in FIG. 12.
However, in the case where the starting motor and the other auxiliary machines are connected through the same belt, position of the tight side and that of the slack side in the belt vary depending on whether it is the time of starting the engine by means of the starting motor (electric rotating machine) or the time of driving the auxiliary machine by means of the engine. Therefore, a method of simply inserting the first auto-tensioner on the belt slack side at the time of driving the auxiliary machines cannot meet the requirements. For example, in the case where the first auto-tensioner is inserted on the slackest side of the belt at the time of driving the auxiliary machines, the first auto-tensioner will be positioned on the belt tight side at the time of starting the engine, resulting in no function as an auto-tensioner.
Furthermore, when operation is switched from driving the engine by means of the starting motor to driving the auxiliary machines by means of the engine, it occurs that the crank pulley begins to rotate, in which rotation of the crank pulley is superior to that of the pulley of the starting motor. Under such situation, a large tension, which has been applied to the belt up to that moment, is sharply reduced to substantially zero, and the belt suffers from being slipped due to the pulley of the starting motor. As a result, another problem exists in that the belt is worn away as well as noise accompanied by the slip is generated. In this connection, to prevent slippage, it may be an idea to pull the pulley under the sate that the belt is wrapped around and increase an initial tension of the belt. To transmit torque at the time of start-up, however, it is necessary to apply an extraordinary large tension. In this case, when such a large belt tension is applied to the pulleys mounted on the other auxiliary machines, it becomes necessary to increase durability or strength of shaft, bearing and other support structure of the auxiliary machine. This results in a further problem of larger size and higher cost of the auxiliary machines.
As a second prior art, the Japanese Patent No. 3129268 discloses a belt drive system as shown in FIG. 13. In this belt drive system, a second auto-tensioner 21 is located on the belt slackest side where slack produced on the belt becomes the largest at the time of starting the engine by means of the starting motor. With this second auto-tensioner 21, a predetermined belt tension is maintained, and it becomes possible to obtain a tension required for transmitting a starting torque on the belt tight side (between the staring motor pulley 13 and the crank pulley 2) at the time of starting an engine. Further, the crank pulley 2 is disposed on the belt tightest side where the tension produced in the belt 7 becomes the largest. Furthermore, the other auxiliary machines are disposed on the tight side of the belt between the crank pulley 2 of the engine and the second auto-tensioner 21 for driving the other auxiliary machines by means of the engine, thereby enabling to prevent the rotary shaft and bearing of the other auxiliary machine from being applied with the maximum belt tension at the time of starting the engine.
However, since rotational power generation source is changed from the starting motor to the engine depending on whether it is the time for starting the engine or the time for driving the auxiliary machines, positions of the tight side and the slack side of the belt 7 relative to the rotational power generation source are changed following the change in rotational power generation source. Thus, a belt tension is most reduced between the crank pulley 2 mounted on the crankshaft and the starting motor pulley 13, and eventually it is possible that a belt slippage occurs at the crank pulley 2 or the starting motor pulley 13.
As countermeasures to the above-mentioned belt slippage, it may be an idea to set the tension of the above-mentioned second auto-tensioner larger. In this case, however, since durability of the belt 7 is remarkably reduced, it is difficult to augment a set tension to a degree of being capable of preventing the belt slippage. Moreover, for the above-mentioned reasons, it is also difficult to preliminarily set an initial tension of the second auto-tensioner 21 to be large taking into consideration slack of the belt 7 due to change with the passage of time or the like.
Further, as a third prior art, the Japanese Patent Publication (unexamined) No. 59555/2001 discloses a belt drive system as shown in FIG. 14. In this belt drive system, the idle pulley 17a of the first stopper-equipped auto-tensioner 17 that is capable of adjusting a belt tension is located between the crank pulley 2 of the engine and the starting motor pulley 13. This prior art is constituted such that position of the idle pulley 17a is stationary at the time of staring the engine by means of the starting motor, while the idle pulley 17a coming to be movable at the time of driving the other auxiliary machine by means of the engine. Thus, position of the idle pulley 17a of the first stopper-equipped auto-tensioner 17 is stationary when starting the engine thereby enabling to maintain an initial tension of the belt with the second auto-tensioner 21. Further, the idle pulley 17a of the first stopper-equipped auto-tensioner comes to be movable at the time of driving the auxiliary machines, thereby enabling to obtain a function as an auto-tensioner.
However, since the second auto-tensioner 21 is positioned on the belt tight side with respect to the staring motor pulley 13 at the time of driving the auxiliary machines, a difference in tension required for driving between the belt tight side and the belt slack side is definitely essential in order to drive the starting motor.
In particular, in the case where the starting motor functions as a generator or in the case where the auxiliary machines are connected in series to the back side of the staring motor and function as a large load at the time of driving the auxiliary machines, the above-mentioned difference in tension comes to be large.
In this case, on the belt slackest side of the crank pulley 2, a tension value of the second auto-tensioner 21 has to be set larger than a necessary set tension value of the first stopper-equipped auto-tensioner 17 to prevent slippage at the crank pulley portion just by a difference in tension required for driving the mentioned starting motor.
Generally, in the case where a plurality of auto-tensioners are provided on one belt, an initial tension of the belt is to be converged with a tension value of the auto-tensioner of which set tension value is the largest among those plural auto-tensioners. Therefore, in the belt electromotive system as shown in FIG. 14, a set tension value of the mentioned second auto-tensioner 21 becomes larger than a set tension value of the first stopper-equipped auto-tensioner 17 by a tension difference required for driving, e.g., the starting motor, eventually resulting in a problem that the belt initial tension cannot be reduced.
Since the above-mentioned conventional system is constituted as described above, it is necessary to set the belt initial tension to be large in order to prevent the belt from occurrence of slippage both at the time of starting an engine and at the time of driving auxiliary machines. As a result, increase in durability or strength of the shaft, bearing and other support structure of the auxiliary machine is essential, eventually resulting in occurrence of a problem of growing in size, increase in weight and cost.